The 3mm Allen nut is now stuck fast and I can’t get my lock grips off the bars. Any suggestions please?

Hammer in a slightly bigger one?

Hammered torx bit, Dremel or drill

+1 for hammering in a torx. Needs be a tight fit but does work. Another option is to cut the bolt with a thin hacksaw blade if the gap is wide enough.

Drill it out. Idealy left hand drill, but not necessary.

Or araldite in a allen key, Then when removed heat up the bolt to weaken the epoxy and pull the blot off.

Drill it out from the other side.

if you own torx bits then try tapping one of them in, you’ll prob need a T10 or T15 bit. Or try a 3/32 imperial allen key or 1/8th.

If you have none of the above then take a cold chisel to the inner edge of the grip and whack it with a hammer – be careful if you have carbon bars.

Edit : drilling it out is a no win situation – for the reason the allen bolt is steel, the lock on end cap is alloy – you do the engineering.

if it comes to that situation then take a hacksaw blade to the split/compression join on the alloy lock on cap

+1 for drilling from the other side. I had a Ti stem bolt snap inside a carbon stem and a small drill did the trick.

T10 too small, T15 too big. +1 for the cold chisel though (thanks Somafunk). Wrapped it in a towel first to limit damage and bob’s your Knob. Good job it was the outer nut only.
Now to see if I can replace the grip end, but I guess I’m looking at new grips.
Thanks for all the speedy replies.

To anyone else with this specific problem.
Use a G-clamp to close the grip enough to take the pressure off the bolt. Then undo

G clamp is a genius idea!

No bother, now that the grip is off the bars the bolt may come out as there is no tension on the underside of the head to stretch the bolt, squeeze the lock-on ring together and see if you can get the bolt out – you’ve got nowt’ to lose.

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Does the clamp have a gap where you could hacksaw the bolt shaft?

G-clamp was a great idea. Even with the grip off the bars I couldn’t turn the nut but a bit of wd40 and a clamp that was way too big for the job and finally the nut is out. (a bolt actually of course)
So glad I didn’t drill or cut because now I only need a new bolt.
Cutting would have been very difficult because although the was a tiny gap it was no wider that a hacksaw blade. Drilling would almost certainly have made a good old mess of the alloy.
As usual a great team effort from STW, thanks all.

Is there enough space to cut a slot and then use a screwdriver?

no need, as you can see the bolt is now out, unless you mean in order to reuse it but I think I’ll just buy another.